The present invention relates generally to processes for making acetic acid; and in particular to a low energy process for making acetic acid by way of carbonylating methanol with carbon monoxide and utilizing at most two distillation columns in the primary purification train
Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the rhodium catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 of Paulik et al. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium along with a halogen containing catalyst promotor as exemplified by methyl iodide. Generally, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled. Paulik et al. disclosed that water may be added to the reaction mixture to exert a beneficial effect upon the reaction rate. Water concentrations greater than about 14 weight percent are typically used. This is the so called xe2x80x9chigh waterxe2x80x9d carbonylation process.
An alternative to the xe2x80x9chigh waterxe2x80x9d carbonylation process is the xe2x80x9clow waterxe2x80x9d carbonylation process as disclosed in U.S. Pat. Nos. 5,001,259; 5,026,908; and 5,144,068. Water concentrations below 14 weight percent and even below 10 weight percent can be used in the xe2x80x9clow waterxe2x80x9d carbonylation process. Employing a low water concentration simplifies downstream processing of the desired carboxylic acid to its glacial form.
It is desirable in a carbonylation process for making acetic acid to minimize the number of distillation operations in order to minimize energy usage in the process. In this respect there is disclosed in U.S. Pat. No. 5,416,237 to Aubigne et al. a process for the production of acetic acid by carbonylation of methanol in the presence of a rhodium catalyst, methyl iodide, and an iodide salt stabilizer. The improvement according to the ""237 patent resides in maintaining a finite concentration of water up to about 10 percent by weight and a methyl acetate concentration of at least 2 percent by weight in the liquid reaction composition and recovering the acetic acid product by passing the liquid reaction composition through a flash zone to produce a vapor fraction which is passed to a single distillation column from which the acetic acid product is removed. The drawback of eliminating distillation stages is that the level of purity of the product suffers. In particular the distillation columns tend to remove high boiling iodides as well as aldehyde contamination products. Both of these impurities impact the commercial desirability of the final product.
Various means for removing iodides are well known in the art. It was discovered by Hilton that macroreticulated, strong acid cationic exchange resins with at least one percent of their active sites converted to the silver or mercury form exhibited remarkable removal efficiency for iodide contaminants in acetic acid or other organic media. The amount of silver or mercury associated with the resin may be from as low as about one percent of the active sites to as high as 100 percent. Preferably about 25 percent to about 75 percent of the active sites were converted to the silver or mercury form and most preferably about 50 percent. The subject process is disclosed in U.S. Pat. No. 4,615,806 for removing various iodides from acetic acid. In particular there is shown in the examples removal of methyl iodide, HI, I2 and hexyl iodide.
Various embodiments of the basic invention disclosed in U.S. Pat. No. 4,615,806 have subsequently appeared in the literature. There is shown in U.S. Pat. No. 5,139,981 to Kurland a method for removing iodides from liquid carboxylic acid contaminated with a halide impurity by contacting the liquid halide contaminant acid with a silver (I) exchanged macroreticular resin. The halide reacts with the resin bound silver and is removed from the carboxylic acid stream. The invention in the ""981 patent more particularly relates to an improved method for producing the silver exchanged macroreticular resins suitable for use in iodide removal from acetic acid.
U.S. Pat. No. 5,227,524 to Jones discloses a process for removing iodides using a particular silver-exchanged macroreticular strong acid ion exchange resin. The resin has from about 4 to about 12 percent cross-linking, a surface area in the proton exchanged form of less than 10 m2/g after drying from the water wet state and a surface area of greater than 10 m2/g after drying from a wet state in which the water has been replaced by methanol. The resin has at least one percent of its active sites converted to the silver form and preferably from about 30 to about 70 percent of its active sites converted to the silver form.
U.S. Pat. No. 5,801,279 to Miura et al. discloses a method of operating a silver exchanged macroreticular strong acid ion exchange resin bed for removing iodides from a Monsanto type acetic acid stream. The operating method involves operating the bed silver-exchanged resin while elevating the temperatures in stages and contacting the acetic acid and/or acetic anhydride containing the iodide compounds with the resin. Exemplified in the patent is the removal of hexyl iodide from acetic acid at temperatures of from about 25xc2x0 C. to about 45xc2x0 C.
So also, other ion exchange resins have been used to remove iodide impurities from acetic acid and/or acetic anhydride. There is disclosed in U.S. Pat. No. 5,220,058 to Fish et al. the use of ion exchange resins having metal exchanged thiol functional groups for removing iodide impurities from acetic acid and/or acetic anhydride. Typically, the thiol functionality of the ion exchange resin has been exchanged with silver, palladium, or mercury.
There is further disclosed in European Publication No. 0 685 445 A1 a process for removing iodide compounds from acetic acid. The process involves contacting an iodide containing acetic acid stream with a polyvinylpyridine at elevated temperatures to remove the iodides. Typically, the acetic acid was fed to the resin bed according to the ""445 publication at a temperature of about 100xc2x0 C.
With ever increasing cost pressures and higher energy prices, there has been ever increasing motivation to simplify chemical manufacturing operations and particularly to reduce the number of manufacturing steps. In this regard, it is noted that in U.S. Pat. No. 5,416,237 to Aubigne et al. there is disclosed a single zone distillation process for making acetic acid. Such process modifications, while desirable in terms of energy costs, tend to place increasing demands on the purification train. In particular, fewer recycles tend to introduce (or fail to remove) a higher level of iodides into the product stream and particularly more iodides of a higher molecular weight. For example, octyl iodide, decyl iodide and dodecyl iodides may all be present in the product stream as well as hexadecyl iodide; all of which are difficult to remove by conventional techniques.
Other impurities in acetic acid made by way of the rhodium catalyzed carbonylation of methanol, notably aldehydes and propionic acid, are likewise known. It is proposed in an article by Watson, The Cativa(trademark) Process for the Production of Acetic Acid, Chem. Ind. (Dekker) (1998) 75 Catalysis of Organic Reactions, pp. 369-380, that acetaldehyde undergoes reduction by hydrogen in the rhodium-catalyzed system to give ethanol which subsequently yields propionic acid. It is postulated that enhanced rhodium catalyzed systems have increased standing levels of rhodium-acyl species which will form free acetaldehydes at a higher rate.
The precise chemical pathway within the methanol carbonylation process that leads to the production of crotonaldehyde, 2-ethyl crotonaldehyde and other permanganate reducing compounds is not well understood. One prominent theory for the formation of the crotonaldehyde and 2-ethyl crotonaldehyde impurities in the methanol carbonylation process is that they result from aldol and cross-aldol condensation reactions that involve acetaldehyde. Substantial efforts have been directed to removing acetaldehyde.
Conventional techniques used to remove acetaldehyde and other carbonyl impurities have included treatment of acetic acid with oxidizers, ozone, water, methanol, amines, and the like. In addition, each of these techniques may or may not be combined with the distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the product acetic acid. Likewise, it is known that carbonyl impurities can be removed from organic streams by treating the organic streams with an amine compound such as hydroxyl amine which reacts with the carbonyl compounds to form oximes followed by distillation to separate the purified organic product from the oxime reaction products. However, this method of treating the product acetic acid adds cost to the process.
There is disclosed in U.S. Pat. No. 5,625,095 to Miura et al. and PCT International Application No. PCT/US97/1871 1, Publication No. WO 98/17619 various methods of removing acetaldehydes and other impurities from a rhodium-catalyzed acetic acid production process. Generally, these methods involve removing undesirable impurities from recycle streams to reduce acetaldehyde concentrations in the system.
There is provided in accordance with the present invention a low energy carbonylation process utilizing in the primary purification train at most two distillation columns. In accordance with the inventive process, the amount of aldehydes in the product stream is preferably controlled by removal of aldehydes from the system or by operating the process such that low levels of aldehyde contaminants and their derivatives, such as organic iodides are generated. Moreover, high boiling iodides are removed by way of a high temperature ion exchange resin such that the product exhibits high levels of purity.
More specifically, there is provided in accordance with the present invention a continuous process for producing acetic acid including:
(a) reacting methanol with a carbon monoxide feed stock in a carbonylation reactor holding a catalytic reaction medium while maintaining in said reaction medium during the course of said reaction at least a finite concentration of from about 0.1 weight percent up to less than 14 weight percent of water together with: (i) a salt soluble in the reaction medium at the reaction temperature in an amount operative to maintain a concentration of ionic iodide in the range of from about 2 to about 20 weight percent effective as a catalyst stabilizer and co-promoter; (ii) from about 1 to about 20 percent methyl iodide; (iii) from about 0.5 to about 30 weight percent methyl acetate; (iv) a rhodium catalyst; and (v) acetic acid. A portion of the reaction medium is withdrawn from the reactor and vaporized in a flashing step. The flashed vapor is distilled to form a liquid acetic acid product stream utilizing up to two distillation columns while providing one or more recycle streams to the reactor. The amount of aldehyde in the liquid acetic acid product stream is optionally controlled by one of three techniques or combinations of these techniques which include: (i) operating the reactor at a total pressure of from about 15 to about 40 atmospheres while maintaining a partial pressure of hydrogen of less than about 6 psia; (ii) maintaining in the reaction medium a concentration of less than about 5 weight percent methyl iodide; and (iii) removing aldehyde impurities from at least one of the recycle streams.
Particularly preferred iodide salts are alkali metal iodide salts such as lithium iodide. The salts may be formed in-situ, for example, by adding lithium acetate or salt forming phosphines including pentavalent phosphine oxides to the reactor. So long as the ionic iodide is measurable by silver titration, minimizes rhodium precipitation and operates to maintain the majority of or at least 50% of the rhodium in the Rh(I) oxidation state at water concentrations of less than 14%, it is a xe2x80x9csaltxe2x80x9d, as defined herein. Salts may be used alone or in combination to maintain the requisite level of ionic iodide. Compare, U.S. Pat. No. 5,817,869 with U.S. Pat. No. 6,031,129, the disclosures of which are incorporated by reference.
Iodides are removed from the liquid acetic acid product residue stream such that the product has an iodide content of less than about 10 ppb iodide. The iodides are removed by one of two processes:
(a) a first process involves contacting the liquid acetic acid product residue stream with an anionic ion exchange resin at a temperature of at least about 100xc2x0 C. followed by contacting the liquid acetic acid product residue stream with a silver or mercury exchanged ion exchange substrate wherein at least one percent of the active sites (i.e., sulfonic acid moieties) of the resin have been converted to the silver or mercury form;
(b) a second process involves contacting the liquid acetic acid product residue stream with a silver or mercury exchanged ion exchange substrate at a temperature of at least about 50xc2x0 C. wherein at least one percent of the active sites of the resin have been converted to the silver or mercury form.
When utilizing an anionic resin, particularly preferred resins include polyvinylpyridine resins and polyvinylpyrrolidone resins. The anionic resins are typically employed at a temperature of at least about 150xc2x0 C.
When a silver or mercury exchanged substrate is used, it is typically a macroreticular, strong acid cationic resin. Temperatures may be from about 60 to about 100xc2x0 C. A minimum temperature of 60xc2x0 C. is sometimes employed while a minimum temperature of about 70xc2x0 C. may likewise be preferred in some embodiments.
In general, when a silver or mercury exchanged strong acid cationic resin is employed typically from about 25% to about 75% of the active sites are converted to the silver or mercury form. Most typically about 50% of the active sites are so converted.
The aldehydes in the system may optionally be controlled by removing aldehydes from the recycle to the reactor by way of, for example, distillation from a condensed recycle stream.
Alternatively the level of aldehyde impurities in the system may be controlled by minimizing the partial pressure of hydrogen or the levels of methyl iodide in the reactor. In particular, at a total pressure in the reactor of 15 to 40 atmospheres absolute a partial pressure of from about 0.1 to about 4 psia of hydrogen may be employed. A partial pressure of hydrogen of from about 1 to about 4 psia may be preferred. Relatively low level of methyl iodide in the reactor may be about 5 weight percent or less. A level of methyl iodide of from about 1 to about 5 weight percent may likewise be employed.
In another aspect of the invention, there is provided an acetic acid made by the process described herein, wherein the product has a propionic acid content of less than about 500 ppm. Typically, the product acid has a propionic acid content of less than about 250 ppm, with less than about 150 ppm being preferred.
Particularly preferred processes are those utilizing a silver-exchanged cationic substrate for removing iodides and relatively low hydrogen partial pressures in the reactor for controlling aldehyde impurities. The product stream in many cases includes organic iodides with a C10 or more aliphatic chain length which need to be removed. Sometimes more than 25% of the iodides present, or even 50%, have an organic chain length of more than 10 carbon atoms.
Decyl iodides and dodecyl iodides are especially prevalent in the absence of heavy ends and other finishing apparatus and are difficult to remove from the product stream as will be appreciated from the data hereinafter appearing. The silver-exchanged cationic substrates of the present invention typically remove over 90% of such iodides; oftentimes the product stream has from 10 to about 1000 ppb total iodide prior to treatment which would make the product unusable for iodide-sensitive applications.
From about 20 ppb to about 750 ppb prior to iodide removal treatment is somewhat typical; whereas the iodide removal treatment is preferably operative to remove at least about 99% of the total iodide present.
In a typical embodiment, iodide removal treatment involves contacting the product with a silver-exchanged sulfonic acid functionalized macroreticular ion exchange resin, wherein the product has an organic iodide content of greater than 100 ppb prior to treatment and an organic iodide contact of less than 10 ppb after contacting the resin.
The following related applications belonging to the Assignee of the present invention are incorporated herein by reference, the pertinent portions of which are further described herein:
U.S. Ser. No. 09/386,708, filed Aug. 31, 1999 of Mark O. Scates et al., entitled xe2x80x9cRhodium/Inorganic Iodide Catalyst System for Methanol Carbonylation Process with Improved Impurity Profilexe2x80x9d; U.S. Ser. No. 09/386,561, filed Aug. 31, 1999 of Hung-Cheun Cheung et al., entitled xe2x80x9cRhodium/Inorganic Iodide Catalyst System for Methanol Carbonylation Process with Improved Impurity Profilexe2x80x9d; and U.S. Ser. No. 09/534,868, filed Mar. 21, 2000 of George A. Blay et al., entitled xe2x80x9cMethod of Removing Organic Iodides from Organic Mediaxe2x80x9d.
The foregoing and further features of the present invention will be further appreciated form the discussion which follows.
Unless otherwise indicated by the context or explicitly, as used herein, xe2x80x9c%xe2x80x9d, xe2x80x9cpercentxe2x80x9d or the like refers to weight percentage. Likewise, the terminology xe2x80x9cppmxe2x80x9d, xe2x80x9cparts per millionxe2x80x9d and the like and xe2x80x9cppbxe2x80x9d refers to parts per million by weight or parts per billion by weight, respectively, unless otherwise defined. The terminology xe2x80x9cactive sitesxe2x80x9d of an ion exchange resin refers to the ion exchange sites available in such a resin. For example, in a cationic ion exchange resin having a cation exchange capacity of 2 meq/g, 2 meq/g constitutes 100% of the active sites, 1 meq/g constitutes 50% of the active sites and so forth.